For a concentric connecting method for precise parts comprising a plurality of members, an example of a conventional nozzle assembly will be described with reference to Japanese Patent Publication No. Hei. 7-10471 (corresponding to U.S. Pat. No. 5,127,156, GB 2,236,359, DE 4,030,320). In FIG. 1 of the aforesaid Publication, a tapered hole (a valve seat) 10c is formed in the inner bottom provided with an orifice 11 of a nozzle body (an outer tubular part) 10, a swirler (an inner tubular part) 12 with a through-hole 12a is installed within the nozzle body 10 while securing a clearance therebetween, the vicinity (on the swirler side) of a fitting portion between the swirler 12 and the nozzle body 10 is pressed by a punch 16 so as to generate a local plastic flow while centering the tapered hole 10c with respect to the through-hole 12a of the swirler 12 by a positioning guide pin 14, and both the parts are connected concentrically by force of the plastic flow thus generated. The swirler 12 is internally formed with a fuel swirling force generating groove for applying swirl to fuel, and fuel is injected out of a fuel injection valve while swirling.
As mentioned in the above prior art, a swirler (an inner tubular part) 12 with a through-hole 12a is installed within the nozzle body 10 while securing a clearance G therebetween, the vicinity of a fitting portion of the swirler 12 is pressed by a punch 16 so as to generate a local plastic flow while centering the tapered hole 10c with respect to the through-hole 12a of the swirler 12 by a positioning guide pin 14, and both the parts are connected concentrically by force of the plastic flow, in such a case, a residual stress due to the plastic flow occurs without fail in the connecting portion.
If a coaxial degree of the inside and outside diameters of the swirler 12 is 0, and a coaxial degree of the inside diameter of the nozzle body 10 and the tapered hole 10c is 0, the residual stress is generated uniformly over the whole circumference, by which ideal concentric connection is attained. However, where either of the parts has deflection, that is, the coaxial degree is not 0, or where the coaxial degree between the outside diameter and the inside diameter of the swirler 12 is not 0, the dimension of the clearance G in the whole circumference is partly varied, so that the residual stress of the whole circumference of the connecting portion becomes unbalanced. Because of this, when the positioning guide pin 14 is removed, the spring back amount in the whole circumference is different due to the unbalance of the residual stress, resulting in deviation of center. As described, in the conventional method, there remains considerably an influence of accuracy of an individual part of the parts on the coaxial degree after connection. In the case of the embodiment in the aforesaid Publication, the coaxial degree after connection is 5.8 .mu.m on the average.
Further, where both the nozzle body 10 and the swirler 12 are formed of a combination of materials that are not subjected to plastic flow, such as a hardening material, it is impossible to connect both the parts making use of plastic flow. Therefore, the method as in the above-described prior art cannot be employed.